1. Field of the Invention
The invention relates to an image forming apparatus such as a printer or a copying machine.
2. Description of Related Art
An image forming apparatus such as a printer or a copying machine is generally provided with density controlling means for automatically adjusting the density of an output image (e.g. a toner image) to a proper density. Particularly, in an image forming apparatus that outputs a toner image of four full colors, in order to obtain a desired color balance, more accurate density control is required of yellow, magenta, cyan and black toner images.
Density detection is effected, for example, by forming a toner image of a particular halftone pattern by area coverage modulation (hereinafter suitably referred to as a “patch image”) on a photosensitive drum (image bearing member), and measuring the amount of reflected light of the halftone pattern on the photosensitive drum by a reflected light amount sensor comprising a light emitting element and a light receiving element. The density of the toner image can be controlled by image forming conditions such as the charging potential of the photosensitive drum, the exposure potential after laser exposure and development bias potential. So, one or a combination of a plurality of these image forming conditions is stepwisely changed to thereby form a plurality of halftone patterns, and the amount of reflected light thereof is measured by the reflected light amount sensor to thereby find image forming conditions under which it is presumed that desired constant density (amount of reflected light) can be obtained. As the reflected light amount sensor, use was made of one using infrared light and capable of estimating the amount of toner on the photosensitive drum irrespective of the color of the toner. The amount of infrared light received by the light receiving element of the reflected light amount sensor is substantially in inverse proportional to the amount of adhering toner, but the amount of adhering toner and the density of the output image are generally not in proportionality relation with each other. However, the amount of adhering toner and the density of the output image can be made to correspond to each other in a one-to-one relationship and therefore, the density of the toner image (output image) can be estimated from the measured value by the reflected light amount sensor.
This density control has a great effect in the stabilization of the dignity of image chiefly comprising a halftone such as a photographic image, and besides, to what degree the interval at which such density control is frequently effected depending on changes such as the fluctuation of the potential of the photosensitive drum, the fluctuation of the developing characteristic and the fluctuation of environment, namely, the time interval at which the density control is effected, is set becomes important.
That is, if the interval is set short and the density control is effected frequently, the dignity of image will be correspondingly stable. Conversely, however, to a user, image formation cannot be done during the density control and therefore, the time during the density control is not only a useless time, but the toner is uselessly consumed by forming a patch image and moreover, there is the demerit that waste toner is unnecessarily increased. If in contrast, the interval is set long, there will be the possibility that the especial density controlling mechanism cannot be used fully effectively and the dignity of image is reduced.
Therefore, heretofore, as an ordinary case, density control has been effected during the closing of the power supply switch of the main body of the image forming apparatus or each time a predetermined number of sheets of image formation is terminated, and as a special case, density control has been effected during the interchange of the photosensitive drum or a developing apparatus.
However, the fluctuation of the potential of the photosensitive drum, the fluctuation of the developing characteristic, the fluctuation of environment, etc. do not always change at a constant rate even if the interval is set to a constant one, and the fluctuation ranges are not constant. Thus, if as is usual, control is effected for each predetermined number of sheets, at one time it will become necessary to effect density control more frequently in order to keep the dignity of image, and at another time unnecessary density control will be effected although the dignity of image will not be reduced even if the interval is made longer. In such case, an increase in the user's waiting time or an increase in toner consumption and waste toner will result as previously described.